1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for adaptively controlling a data transmission rate according to a channel status, i.e., a signal-to-interference ratio, in which a delay time is used in determining a forward data rate to be requested.
2. Discussion of the Related Art
A wireless communication terminal is a portable terminal for wireless communication, such as a PCS, a PDA, a smart phone, or a wireless LAN terminal. In a wireless communication system such as the IMT-2000 standard, which enables high-speed data transmission, channel status fluctuates rapidly over time as a result of channel fading and other factors. Therefore, the current channel status is an important factor in determining the data rate in a wireless communication system and is particularly problematic in asymmetrical data links such as the downloading of a data stream on a forward traffic channel.
In the downlink of a wireless communication system, a base station uses a forward channel to transmit a pilot signal, which is received by a wireless communication terminal. The power level of the received pilot signal is measured to determine a current channel status, so that a forward data rate appropriate for the current channel status can be determined. Here, the channel status is based on a measurement of a signal-to-interference ratio (SIR), and a lookup table is used to determine a forward data rate for request based on the obtained SIR value.
The requested forward traffic channel data rate is mapped into a four-bit data rate control (DRC) value as specified by the forward traffic channel MAC protocol. The wireless communication terminal uses a DRC channel to transmit the DRC value to the base station, which thereafter transmits data on the forward traffic channel in accordance with the requested forward data rate. In doing so, DRC symbols occupy at least one slot of the DRC channel corresponding to a DRC length beginning with the first slot of the DRC channel following the pilot signal transmission on the forward traffic channel. Subsequently, data transmission on the forward traffic channel is performed at the requested data rate, beginning from the next slot following the transmission of the DRC symbols. Therefore, conventionally, there is a delay time between pilot signal transmission and data transmission, which corresponds to the DRC length plus one slot.
Meanwhile, the channel status dynamic may produce a change in the channel status, occurring during the above-mentioned delay time, such that data transmission from the base station cannot reflect the most current channel status at the wireless communication terminal. In particular, if the channel status becomes degraded during the delay time, the requested forward data rate, which was determined based on the channel status at the time of pilot signal reception, may be too high. Accordingly, there is a need for an adaptive DRC method in which this potential change in channel status is considered.